The invention relates to a DC/DC conversion circuit comprising a transformer with a primary winding, a secondary winding, and an auxiliary winding; a switching transistor which forms a series circuit with the primary winding between first and second DC voltage input connection terminals; a start circuit which is connected to a control electrode of the switching transistor for switching on the latter when the DC voltage input connection terminals are powered; a control transistor connected to the control electrode of the switching transistor for switching off the switching transistor in dependence on a current flowing in said series circuit during operation; and a voltage control circuit connected to the auxiliary winding and the control transistor for controlling an output power supplied at the secondary winding of the transformer during operation through switching-on of the switching transistor in a controlled manner in dependence on the value of a current flowing in the primary winding.
A conversion circuit of this kind, also referred to as switch mode power supply or self-oscillating power supply (SOPS) in the literature, is known per se from U.S. Pat. No. 5,675,479.
It is known that the switching losses in the switching transistor, which is usually a transistor of the field effect type, for example a metal oxide semiconductor transistor (MOST), increase quadratically with the voltage across the main conduction path of the switching transistor. This is the drain-source voltage in the case of a MOST. A reduction of the drain-source voltage at the moment of switching-on of the switching transistor accordingly has a major influence on the reduction of the switching losses.
It is also known that the frequency with which the switching transistor is switched in a SOPS increases as the output power at the secondary side of the transformer decreases, and will be theoretically infinite for an output power equal to zero. Since the switching losses of a MOST increase linearly with the switching frequency, however, thermal problems may arise in the switching transistor at lower output powers. In particular, the idle or standby power of the circuit is comparatively high, which is undesirable, for example for reasons of economy.
To avoid thermal overloading of the switching transistor, a suppression circuit is used in the conversion circuit proposed by the cited U.S. Pat. No. 5,675,479 for reducing the oscillation frequency when the generated output power is low. The suppression circuit is switched off when the output power is high.
The switching-on and -off of the suppression circuit gives rise to a jump in the oscillation frequency in this known conversion circuit. This renders an accurate control of the output power supplied at the secondary side of the transformer more difficult because the switching-on and -off of the suppression circuit is subject to a certain hysteresis.
The invention accordingly has for its object to improve a DC/DC conversion circuit of the kind mentioned in the opening paragraph such that, at comparatively low output powers, i.e. low as compared with the nominal power which can be supplied by the conversion circuit, the switching frequency of the switching transistor is also low and varies gradually with the output power given off.
According to the invention, this object is achieved in that the conversion circuit is designed such that during operation a voltage required for switching the switching transistor on is built up on the control electrode thereof in a time period whose length is inversely proportional to the value of the output power supplied to the secondary winding.
After the switching transistor has been switched on through the start circuit, a current will flow from the input connection terminals through the series circuit of the switching transistor and the primary winding of the transformer. This current will have a rising gradient because energy is built up in the transformer. When the current reaches a preset peak value, the control transistor is energized, which transistor is connected such that the voltage at the control electrode of the switching transistor is reduced, so that the switching transistor switches itself off. The energy stored in the transformer is now given off to a load connected to the secondary side of the transformer. Subsequently, a self-oscillatory effect arises at the primary side of the transformer under the influence of capacitive and inductive action, whereby an AC voltage is induced in the auxiliary winding. This voltage is coupled to the control electrode of the switching transistor through the voltage control circuit.
The conversion circuit according to the invention utilizes the effect that the induction voltage in the auxiliary winding for a higher output power at the secondary side of the transformer is higher than for a lower output power. The voltage at the control electrode for switching-on of the switching transistor can accordingly be built up more quickly in the case of a comparatively high output power than in the case of a lower output power. As a result, the switching transistor will be switched with a comparatively lower switching frequency in the case of lower output powers than in the case of higher output powers of the conversion circuit. It will be obvious that the switching frequency is made gradually dependent on the output load in a comparatively simple manner, i.e. in that the switching frequency is reduced when the output load is reduced, whereby the switching losses in the switching transistor are limited.
A jump in the switching frequency is effectively avoided by the solution according to the invention. The output power of the circuit, i.e. the output voltage, can still be well controlled also for comparatively low output powers thanks to the absence of hysteresis effects.
In a preferred embodiment of the conversion circuit according to the invention, the control electrode of the switching transistor is capacitively loaded by means of at least one element with a capacitive action such that the voltage required for switching-on of the switching transistor is built up in said element during operation.
This embodiment of the conversion circuit according to the invention is characterized by the simplicity of its circuit technology, i.e. the circuit can be realized essentially with only a few discrete components without complicated integrated control circuits or the like.
It was found that the switching transistor switches on at or adjacent a first minimum in the voltage across the main conduction path of the switching transistor, i.e. the drain-source voltage in the case of a MOST, given a suitable dimensioning of the voltage control circuit and the capacitive load on the control electrode of the switching transistor, the gate in the case of a MOST.
At comparatively high output powers, i.e. at a comparatively high voltage induced in the auxiliary winding, this will be the first minimum in the voltage across the main conduction path of the switching transistor. If the output load is comparatively low, however, it will take some time before the voltage at the control electrode is sufficiently high for switching the switching transistor on again, owing to the capacitive load on the control electrode of the switching transistor and the lower induction voltage in the auxiliary winding. The switching transistor will then switch on, for example, at the second, third, or subsequent minimum in the voltage across the main conduction path.
Switching-on of the switching transistor when the voltage across it shows a minimum, i.e. is in a trough, also contributes to a limitation of the switching losses.
In a practical embodiment of the conversion circuit according to the invention, said at least one element with capacitive action is formed by a capacitor which is connected to the control electrode of the switching transistor and which forms part of the start circuit, a coupling capacitor by which the voltage control circuit is connected to the control electrode of the switching transistor, and the inherent gate-source capacitance of the switching transistor itself. The exact values of the various components obviously depend on the properties of the switching transistor and the conversion circuit themselves and may be determined in a comparatively simple manner by those skilled in the art.
In a yet further embodiment of the conversion circuit according to the invention, the output voltage at the secondary side of the transformer may be readily adjusted in that the voltage control circuit comprises a rectifier circuit connected in parallel with the auxiliary winding for applying a bias voltage to the control transistor during operation.
The moment at which the control transistor switches off the switching transistor can be dynamically varied in dependence on the value of the bias voltage, and so can the value of the output voltage generated in the secondary winding as a result.
In a still further embodiment of the conversion circuit according to the invention, provision is made for an optical coupling circuit coupled to the control transistor and the rectifier circuit for influencing the bias voltage applied to the switching transistor during operation. The output voltage can thus be adjusted via the optical coupling circuit (referred to as xe2x80x9coptocouplerxe2x80x9d in the literature), with electrical separation from, for example, the secondary winding of the transformer.